The present invention relates to a fixing device for a dry process copier which is selectively operable in a full-color copy mode and a black-and-white copy mode.
A full-color copier has customarily adopted a procedure wherein an original document is separated in color by blue, green and red filters to form three different electrostatic latent images one after another on a photoconductive element, the individual latent images are developed in colors which are complementary to the colors of the filters, i.e., yellow, magenta and cyan, and the developed images are transferred one upon another to a single paper sheet in register with each other. A problem with this kind of procedure is that when yellow, magenta and cyan toners are simply superposed one upon another, it is difficult to render black and colors in a dark area faithfully due to, among others, spectral reflectance characteristics particular to color toners. To enhance efficient black-and-white mode copying with such a full-color copier, it is extensively practiced to use a black developing unit in addition to yellow, magenta and cyan developing units. Specifically, in a full-color copy mode, the black developing unit develops a latent image which is formed on a photoconductive element by a yellow filter, while the other developing units develops their associated latent images as mentioned above. The resulting yellow, magenta, cyan and black toner images are transferred to a paper sheet one upon another to produce a full-color copy. With this kind of copier, a black-and-white copy which is predominant over a color copy is produced by a single sequence of exposing, developing and transferring steps without using the filters and, therefore, with high efficiency.
A fixing device for a copier having a black-and-white copying capability only and extensively used today has a rigid fixing roller and an elastic roller which is pressed against the fixing roller. The fixing roller is made up of a metal core and a coating of Teflon or similar parting material provided on the core. A heat source is accommodated in the fixing roller. A paper sheet carrying a non-fixed toner image thereon is fed to a nipping section which is defined between the fixing roller and the elastic roller, whereby the toner image is firmly fixed on the paper sheet by heat. This type of fixing device using heat is operable over a long time and with high reliability. When it comes to a full-color copier of the above-described type, however, the surface property and parting ability particular to Teflon is not sufficient to eliminate offsets, jams and irregular images because the toner layer will be composed of yellow, magenta and cyan toners with or without a black toner. With a color copier operable in a full-color copy mode only, it is known that quality images are attainable by implementing a fixing roller as a roller which is coated with silicon rubber or similar elastic material, pressing a rigid roller against the fixing roller, and causing an image surface of a paper sheet into contact with the surface of the roller coated with an elastic material.
The toner layer is thicker in a full-color copy mode than in a black-and-white copy mode. Hence, full-color copying needs two or three times greater amount of heat than black-and-white copying for the fixation of a toner image on a paper sheet. In the light of this, a heat source is sometimes disposed not only in a fixing roller but also in a pressing roller. However, since a copier is powered by an ordinary commercial power source, it is necessary that the total power consumption of a copier be less than 100 volts and 15 amperes, i.e., the wattage allocated to a heat source cannot be increased beyond a certain limit. Some approaches may be contemplated for increasing the amount of heat to be applied to a paper sheet and a toner while maintaining the capacity of a heat source constant: (a) elevating the fixing temperature, (b) increasing the nipping width, and (c) lowering the linear speed. However, the approach (a) is apt to cause silicon oil or similar parting agent applied to the roller surface to fume smoke or cause the temperature inside the machine to elevate, bringing about adverse influences and even dangerous conditions. The approach (b) cannot provide a twice or three times greater nipping width than in a black-and-white copy mode, unless the pressure exerted by the pressing roller is increased. The approach (c) which lowers the linear speed and thereby increases the passing time of a paper sheet through a nipping section is most practical and easy.
A fixing device to which the present invention pertains is a heat roller type fixing device for a dry process copier which is selectively operable in a black-and-white copy mode and a full-color mode. This kind of fixing device, therefore, has to have a long life, not to speak of the ability to desirably fix both a black-and-white copy and a full-color copy. The fixing device using a fixing roller coated with an elastic material and feasible for full-color copies only is not sufficient in the service life aspect and not cost-effective for a copier which is mostly used to produce black-and-white copies. It is necessary that the amount of heat to be applied to a paper sheet and a toner at the time of fixation be switched to an optimum value which differs from black-and-white copying to full-color copying. When such a changeover is effected by changing the nipping width, an arrangement for changing the pressing force will become disproportionately complicated and bulky.